1. Field of the Invention
The present invention relates to a sheet-shaped material positioning device which, by conveying a sheet-shaped material and making a conveying direction leading end portion of the sheet-shaped material abut a pair of positioning pins, corrects the conveying direction position and the tilting with respect to the conveying direction (i.e., the rotational direction in the plane of the sheet-shaped material), so as to position the sheet-shaped material at a predetermined position.
2. Description of the Related Art
Techniques (and printing plate precursor exposure devices) have been developed which utilize a sheet-shaped material (and, in particular, a printing plate precursor) in which an image recording layer is provided on a support), and which directly record an image onto the image recording layer of the printing plate precursor by using a laser beam or the like. With such a technique, an image can be rapidly recorded onto a printing plate precursor. The term printing plate precursor encompasses photopolymer printing plate precursors, thermal type printing plate precursors, silver salt diffusion transfer type printing plate precursors, and the like.
In an automatic printing plate precursor exposure device using the technique of recording an image onto a printing plate precursor, while rotating a rotating drum at high speed (main scanning) in a state in which a printing plate precursor is trained around the peripheral surface of the rotating drum, an exposure head is moved along the axial direction of the rotating drum (subscanning). An image is thereby recorded on the printing plate precursor.
Among this type of automatic printing plate precursor exposure device, there are those in which one end of the printing plate precursor is fixed to the rotating drum by a clamp mechanism having opening/closing type chucks which are fixed to the rotating drum or which are provided so as to be able to be attached to and removed from the rotating drum.
In this clamp mechanism, the chucks are set in an open state (a state in which a gap is formed between the chucks and the rotating drum), and the printing plate precursor is inserted in from a direction tangential to the rotating drum. At the point in time when the printing plate precursor is positioned at a predetermined position, the chucks are set in a closed state (a state in which the chucks and the rotating drum nip the printing plate precursor). The printing plate precursor is thereby held.
Thereafter, while rotating the rotating drum, the printing plate precursor is trained onto the peripheral surface of the rotating drum. Finally, by holding the training direction trailing end portion by chucks for the trailing end, the printing plate precursor can be positioned at a predetermined position on the rotating drum.
In holding the printing plate precursor by the chucks in this way, the printing plate precursor must be positioned precisely. Thus, conventionally, a pair of positioning pins, which are commonly used for printing plate precursor of all sizes (printing plate precursors of all transverse direction dimensions), are projected out on the rotating drum, and the printing plate precursor is conveyed along a guide toward the pair of positioning pins.
Due to the printing plate precursor abutting a positioning member (the pair of positioning pins or a plate for positioning or the like) while being conveyed, the conveying direction position and the tilting of the printing plate precursor are corrected. Moreover, while this state is maintained, by conveying the printing plate precursor in the transverse direction and making the printing plate precursor abut a positioning pin for transverse direction abutment, the center of the printing plate precursor can be made to correspond with, for example, the axial direction center of the rotating drum. Note that this positioning in the transverse direction may be carried out by using one end portion of the printing plate precursor as a reference.
At the point in time when the above-described positioning is completed, the printing plate precursor is held by the chucks, and can be precisely trained around the rotating drum.
However, because the positioning members for the conveying direction correspond to all sizes of printing plate precursors, these positioning members are disposed at an interval (pitch) such that they abut even the smallest sized printing plate precursor at two points. Thus, when the largest sized printing plate precursor is conveyed, the ratio of the interval (pitch) of the pair of positioning pins with respect to the width of the large-sized printing plate precursor is small. It is easy for the printing plate precursor to shake, and there are cases in which the printing plate precursor cannot be positioned accurately with respect to the conveying direction.
FIG. 9 illustrates an experimental example at the time of positioning a large-sized printing plate precursor 200 whose width is 1160 mm, by a pair of positioning pins 202 whose pitch dimension is 380 mm. The pitch dimension of the positioning pins 202 is set so as to be able to correspond to the width (400 nm) of the smallest-sized printing plate precursor.
Here, an error of a maximum of about 15 μm can arise at one of the positioning pins 202. (The imaginary line in FIG. 9 shows the correct position.)
In consideration thereof, when the large-sized printing plate precursor 200 is conveyed with the center as a reference and is positioned by abutting the pair of positioning pins 202, if the error is the maximum (15 μm), the following equation is established.15×10−3/380=x/(1160/2+380/2)  (1)By varying the equation in order to obtain x, the equation becomesx=15×10−3×(1160/2+380/2)/380  (2)Carrying out computation results inx≈30.4 μm  (3)
If the pair of positioning pins 202 were disposed at a pitch of 650 mm, carrying out the same computation would result in a value of about 20.9 μm.
From this, it can be understood that the larger the ratio of the pitch dimension P of the pair of positioning pins 202 with respect to the transverse direction size W of the printing plate precursor (W/P<1), the better the precision.
However, it becomes extremely complicated when more than one pair of positioning pins is aligned on the same line. When the line connecting the axes of the positioning pins meanders (zigzags), it conversely becomes a cause of deterioration of the positioning accuracy of printing plate precursors of respective sizes. Further, because the edges of the printing plate precursor itself are not necessarily rectilinear, it is desirable to provide a pair of pins at a pitch which is optimal for all sizes.